A core in a boiling reactor normally comprises several hundred fuel assemblies. These are arranged vertically in the core and have an at least substantially square cross-section. Each fuel assembly consists of a bundle of fuel rods which are surrounded by a fuel channel. The core also comprises a large number of control rods, which may each comprise four vertical blades arranged in a cruciform configuration and provided with a neutron absorber material. The fuel assemblies are normally arranged in a symmetrical lattice, with each fuel assembly included in two rows of fuel assemblies, located perpendicular to each other, and the control rods normally being arranged with each one of their absorber blades between two fuel assemblies located in the same row, so that each control rod together with four fuel assemblies, arranged around its blades, forms a unit, in the following called control rod unit, having an at least substantially square cross-section and so that the control rod units are arranged in a symmetrical lattice, with each control rod unit included in two rows of control rod units located perpendicular to each other.
The present invention relates to the operation of a nuclear reactor of the type described above.
When the burnup in a reactor has progressed so far that the smallest acceptable core reactivity margin has been attained, a partial recharging of fuel is carried out. By suitably balancing the amount of fuel to be replaced as well as the fissile enrichment of the replacement fuel, an excess reactivity is effected which permits a certain energy output until the next refuelling occasion. In connection with refuelling, when the reactor is shut down, an exchange of control rods is also carried out, if necessary, in which control rods the reactivity worth (by consumption of neutron absorber) has dropped to a predetermined value, or which control rods exhibit defects. Normally, the exchange of control rods is carried out if the reactivity worth in one-fourth of the length of the control rod has dropped to 90% of the original reactivity worth. By reactivity worth of a control rod is meant that change in reactivity which follows the complete insertion of a fully withdrawn control rod in a critical reactor under given conditions.
When exchanging control rods, control rods existing in the reactor at the time of the exchange are replaced, according to the state of the art, by control rods having the same reactivity worth as that of the existing ones in their original state. In this way, the behaviour of the reactor is maintained unchanged.
The control rods in a boiling reactor bind a certain amount of reactivity in the reactor. Together with burnable neutron absorbers, for example digadolinium trioxide, in the fuel, a smallest permissible shutdown margin is obtained for a given core. If it is desired to achieve longer fuel cycles, i.e., a longer operating time between two refuellings, or a larger energy output in the reactor, the shutdown margin in a core with existing control rods has to be dimensioned using an increased quantity of burnable neutron absorber. Such use of an increased quantity of burnable absorber results in reactivity losses because of increased reactivity penalty of residual burnable absorber at the end of a fuel cycle (such as because of an increased content of isotopes of gadolinium with a low absorption cross-section for neutrons). Thus, an increased use of a burnable absorber enables a greater flexibility regarding the design of the core with reactor fuel but at the same time results in increased losses because of the reactivity penalty of residual neutron absorber.